Process for the production of alkenyl carboxylates

ABSTRACT

The present invention provides a process for the production of an alkenyl carboxylate by reacting an alkene, a carboxylic acid and a molecular oxygen-containing gas in a reaction zone in the presence of a catalyst at an elevated reaction temperature, T, to produce an outlet stream from the reaction zone comprising alkenyl carboxylate and oxygen, and wherein in said process the catalyst is contacted with the alkene, at a partial pressure, P, optionally in the presence of the carboxylic acid, and the outlet stream comprises less than 2 vol % oxygen, the improvement comprises reducing the partial pressure of the alkene and/or reducing the reaction temperature so as to suppress formation of benzene and/or suppress inhibition of the catalyst.

This application is the U.S. National Phase of International ApplicationPCT/GB2003/004220, filed 29 Sep. 2003, which designated the U.S.PCT/GB2003/004220 claims priority to British Application No. 0223215.5filed 7 Oct. 2002. The entire content of these applications areincorporated herein by reference.

The present invention relates to a process for the production of alkenylcarboxylates, and, in particular to a process for the production ofvinyl acetate.

Vinyl acetate is generally prepared commercially by contacting ethyleneand acetic acid with molecular oxygen in the presence of a catalystactive for the production of vinyl acetate. The process may be carriedout in either a fixed bed or a fluid bed reactor. A process employing afixed catalyst bed is described, for example, in EP-A-0845453. Processesemploying a fluidised catalyst bed are described, for example, inEP-A-0672453, EP-A-0685449, EP-A-0685451, EP-A-0985655 and EP-A-1008385.For example, EP-A-0 672 453 describes a process for the fluid bedproduction of vinyl acetate from ethylene, acetic acid and a molecularoxygen-containing gas in the presence of a promoted palladium catalyst.

Catalysts active for the production of vinyl acetate may typicallycomprise a Group VIII metal, such as palladium; a co-promoter, such asgold, copper, cerium or mixtures thereof; and, optionally, aco-promoter, such as potassium acetate. For example, catalysts activefor the production of vinyl acetate are described in GB 1 559 540; U.S.Pat. No. 5,185,308 and EP-A-0672453 the contents of which are herebyincorporated by reference. EP-A-0672453, for example, describespalladium containing catalysts and their preparation for fluid bed vinylacetate processes.

A commercial vinyl acetate process is generally operated as a continuousprocess. Ideally, the vinyl acetate process is started up smoothly.However, numerous problems (process upsets) can occur during both thestart-up and operation of the process. In addition to any processupsets, a commercial vinyl acetate process also has planned shut-downs,for example, for the periodic maintenance of the plant and/orreplacement of deactivated catalyst with fresh catalyst.

When a process upset occurs in the production of vinyl acetate fromethylene, acetic acid and molecular oxygen, the molecularoxygen-containing gas feed to the reaction zone is usually shut-off. Inaddition, it may or may not also be desirable to shut off the aceticacid feed to the reaction zone. Irrespective of whether or not theacetic acid feed is shut-off, the catalyst will be exposed to ethylenein the substantial absence of molecular oxygen. Typically, during aplanned shut-down, the molecular oxygen-containing gas feed to thereaction zone is shut-off prior to shutting-off the acetic acid feed.The catalyst will then be left exposed to ethylene in the substantialabsence of molecular oxygen.

It has been found that where catalysts suitable for use in theproduction of alkenyl carboxylates, such as vinyl acetate, are exposedto alkene in the absence or substantial absence of molecular oxygen thecatalyst shows unexpectedly low activity on starting-up or restarting ofthe process. Due to this unexpected loss in catalytic activity (catalystinhibition), the production rate is low and significantly lower thanexpected.

Furthermore, as a result of this reduction in catalytic activity,molecular oxygen introduced in to the reactor on starting-up or ofrestarting the process may not be consumed thereby creating anaccumulation of unreacted molecular oxygen in the reactor and increasingthe risk of explosion.

In addition, it has also been found that where alkenyl carboxylatecatalysts are exposed to an alkene, optionally in the presence of acarboxylic acid, at relatively low levels of molecular oxygen, benzeneproduction may occur. Benzene has a similar boiling point to vinylacetate and therefore separation of the two components is difficult toachieve. Undesirably, therefore, the vinyl acetate product may containunacceptable levels of benzene. It is therefore desirable to reduce oreliminate the formation of benzene in the production of vinyl acetate.In particular, it is desirable to produce a vinyl acetate productcomprising less than 100 ppb benzene.

Thus, there remains a need for an improved process for the production ofalkenyl carboxylates, such as vinyl acetate. In particular, there is aneed for a process in which catalyst inhibition and/or benzene formationare suppressed.

Thus, according to the present invention there is provided a process forthe production of an alkenyl carboxylate by reacting an alkene, acarboxylic acid and a molecular oxygen-containing gas in a reaction zonein the presence of a catalyst at an elevated reaction temperature, T, toproduce an outlet stream from the reaction zone comprising alkenylcarboxylate and oxygen, and wherein in said process the catalyst iscontacted with the alkene, at a partial pressure, P, optionally in thepresence of the carboxylic acid, and the outlet stream comprises lessthan 2 vol % oxygen, the improvement comprises reducing the partialpressure of the alkene and/or reducing the reaction temperature so as tosuppress formation of benzene and/or suppress inhibition of thecatalyst.

Typically, the partial pressure of alkene, P, such as ethylene, in thereaction zone is at least 0.3 bar or greater, such as at least 1 bar,for example, at least 2 bar.

The partial pressure of alkene in the reaction zone is suitably reducedto at least 50% less than P. Typically, P is at least 2 bar, and thepartial pressure of alkene in the reaction zone is reduced to less than1 bar. Preferably, the partial pressure of alkene in the reaction zoneis reduced to approximately 0 bar by removing substantially all thealkene from the reaction zone when low levels of molecular oxygen arepresent. In one embodiment of the present invention, reducing thepartial pressure of alkene comprises removing all reactant gases (i.e.alkene, optional carboxylic acid, and any oxygen present) from thereaction zone, for example by purging the reaction zone with an inertgas, such as nitrogen.

The reaction is typically carried out at a temperature, T, of at least100° C., such as at least 140° C.

The reaction temperature is preferably reduced to at least 20° C. belowT, such as to at least 50° C. below T. More preferably, the reactiontemperature is reduced to below 100° C., such as to 50° C. or lower, forexample, to ambient temperature, such as approximately 20° C.

Generally, in the manufacture of an alkenyl carboxylate, such as vinylacetate, operating at steady-state conditions, the concentration ofoxygen in the outlet stream from the reaction zone is greater than 2 vol%. However, during start-up, shut-down or process upsets theconcentration of oxygen in the outlet stream may be at a low level, thatis less than 2 vol %, such as 0 to 0.5 vol % or 0 to 0.2 vol %.

In general, the amount of benzene produced on exposure of the catalystto the alkene in the presence of low levels of oxygen will vary,depending on, for example, the exact oxygen concentration in the outletstream, the reaction temperature, the specific catalyst used, the alkenepartial pressure and the total reaction pressure. In addition, theamount of benzene produced will depend on the period of time for whichthe catalyst is exposed to the alkene in the presence of low levels ofoxygen.

In the present invention, the catalyst will be exposed to an alkene (andoptionally carboxylic acid) in the presence of low levels of oxygen fora period of time (period of contact), Z, before the partial pressure ofthe alkene and/or the reaction temperature are reduced. Generally, theperiod of contact, Z, should be minimized.

For example, where the catalyst is exposed to alkene at low levels ofoxygen for short periods of time at relatively low reactiontemperatures, only relatively small amounts of benzene may be producedwhich do not significantly affect product quality.

However, higher amounts of benzene may be expected to be produced wherethe oxygen concentration in the outlet stream is significantly lowerthan 2 vol %, such as 0 to 0.5 vol %, and/or where the catalyst isexposed to alkene at low levels of oxygen for longer periods of time.

Benzene formation may be determined by any suitable method known in theart, for example, such as gas chromatography and/or mass spectrometry.The amount of benzene produced may be measured, for example, in theoutlet stream directly at the outlet of the reaction zone and/or at apoint downstream of the outlet from the reaction zone, such as in thefinal vinyl acetate product.

It has also been found that if the oxygen concentration in the outletstream is reduced significantly below 2 vol %, such as to 0.5 vol % orless, the catalyst becomes inhibited. Catalyst employed in a process forthe production of alkenyl carboxylate prior to a shut-down or a processupset will have become inhibited if it shows more than a 10% loss inactivity on resumption of normal operating conditions of the processcompared to its activity immediately prior to the shut-down/processupset.

It has been found that the extent to which the catalyst becomesinhibited is dependent on the time (period of contact) over which thecatalyst is exposed to alkene, and optionally carboxylic acid, in thesubstantial absence of oxygen.

The activity of a catalyst may be determined by any suitable methodknown in the art, for example, by analysis of the amount of productproduced using a suitable analytical techniques, such as gaschromatography and/or mass spectrometry. Typically, in a fluid bedprocess for the manufacture of vinyl acetate, the production rate ofvinyl acetate is suitably determined by calculating the amount of vinylacetate product produced per unit catalyst per unit time. For example,the space-time yield may be measured as the production of vinyl acetatein grammes of vinyl acetate produced per kilogram of catalyst per hour(gVA/kg-cat/hr).

The present invention also provides a process for the production of analkenyl carboxylate in which an alkene, a carboxylic acid and amolecular oxygen-containing gas are contacted in a reaction zone at anelevated temperature, T, in the presence of a catalyst having acatalytic activity y, comprising a Group VIII metal, a promoter and anoptional co-promoter, characterised in that where during the course ofsaid process, the catalyst is contacted with the alkene, optionally inthe presence of the carboxylic acid, and in the substantial absence ofthe molecular oxygen-containing gas, the period of contact, Z, betweenthe catalyst and the alkene, and optional carboxylic acid isinsufficient to reduce the catalytic activity by more than 10% of y.

In a preferred embodiment of the process of the present invention, thealkenyl carboxylate is vinyl acetate. Thus, the present inventionaccordingly provides a process for the production of vinyl acetate inwhich ethylene, acetic acid and a molecular oxygen-containing gas arecontacted in a reaction zone at an elevated temperature, T, in thepresence of a catalyst having a catalytic activity y, comprising a GroupVIII metal, a promoter and an optional co-promoter, characterised inthat where during the course of said process, the catalyst is contactedwith ethylene, optionally in the presence of acetic acid, in thesubstantial absence of the molecular oxygen-containing gas, the periodof contact between the catalyst and ethylene, and optional acetic acidis insufficient to reduce the catalytic activity by more than 10% of y.

The exact degree of inhibition of the catalyst may also be dependentupon factors other than the oxygen concentration and the period ofcontact, Z, such as the specific nature of the catalyst employed, thesensitivity of the catalyst to the alkene and carboxylic acid, thenature of the reactant(s) to which the catalyst is exposed and theirpartial pressures, and also the reaction temperature. Generally,however, the period of contact, Z, between the catalyst and alkene (andoptionally carboxylic acid) in the substantial absence of oxygen shouldbe minimized. Suitably, in the production of vinyl acetate using apromoted Group VIII metal, such as palladium, the period of contact, Z,of the catalyst with ethylene or ethylene and acetic acid, in thesubstantial absence of molecular oxygen, is in the range [>0 to 18]hours, preferably, in the range, [>0 to 12] hours and more preferably,in the range [>0 to 6] hours.

Inhibition of the catalyst may be greater where the catalyst is exposedto both alkene and carboxylic acid, in the substantial absence ofmolecular oxygen. Thus, where the catalyst is contacted with alkene andcarboxylic acid, such as ethylene and acetic acid, in the substantialabsence of molecular oxygen, the period of contact is preferably lessthan the period of contact with alkene alone. Suitably, therefore, theperiod of contact, Z, is in the range [>0 to 12] hours, preferably, inthe range [>0 to 6] hours.

Catalyst inhibition may also be at least partially mitigated by reducingthe partial pressure of carboxylic acid in the reaction zone, even ifthe partial pressure of alkene is not reduced or the reactiontemperature is not reduced. Preferably, however, to avoid or mitigatecatalyst inhibition and simultaneously to avoid or mitigate benzeneproduction, the partial pressure of alkene is reduced and/or thetemperature is reduced.

By minimising the contact time of the catalyst with the alkene or alkeneand carboxylic acid, in the substantial absence of molecular oxygen,reduction in catalytic activity can be avoided or at least mitigatedthereby avoiding prolonged start-up periods and/or reducing the timetaken before the process recovers fill production rates after ashutdown. In particular, fluid bed processes may be run at highernominal molecular oxygen levels in the reaction zone than in fixed bedprocesses. Thus, for example, on start-up of a fluid bed process, inwhich the catalyst is, prior to the introduction of the molecularoxygen-containing gas, contacted with alkene or alkene and carboxylicacid for a prolonged period of time, the catalyst loses activity andtherefore the molecular oxygen introduced into the reaction zone isunconsumed, leading to high levels of molecular oxygen in the reactionzone outlet and an increased risk of explosion. By employing the processof the present invention unsafe operation is mitigated.

Unless otherwise stated all measurements of composition by percentagethroughout this specification are measurements in terms of percentage byvolume. The volume of molecular oxygen in the outlet stream from thereaction zone, as used herein, is measured on a “dry-gas” basis i.e.after removal of condensables that may be present in the outlet streamat the exit of the reaction zone.

Typically, the production of alkenyl carboxylate such as vinyl acetateis carried out heterogeneously with the reactants being present in thegas phase or as a mixture of gas and liquid phases. The process of thepresent invention may be carried out as a fixed bed or a fluid bedprocess, preferably, a fluid bed process.

The alkene may be any suitable alkene or a mixture of alkenes, but ispreferably a C₂-C₄ alkene, such as ethylene.

The alkene may be fed in substantially pure form or admixed with othermaterials, such as, for example, other alkenes or hydrocarbons, hydrogenor inert materials. For example, where the alkene is ethylene, theethylene may be fed in substantially pure form or may be fed admixedwith one or more of nitrogen, methane, ethane, carbon dioxide, water inthe form of steam, hydrogen and C₃/C₄ alkenes or alkanes.

The alkene may comprise fresh and/or recycle alkene.

The fresh and recycle alkene, for example, ethylene, may be introducedinto the reaction zone either as separate feed streams or as a singlefeed stream comprising both fresh and recycle alkene.

The carboxylic acid may be any carboxylic acid or a mixture ofcarboxylic acids, but is preferably a C₂-C₄ carboxylic acid, such asacetic acid.

The alkenyl carboxylates that may be produced in the process of thepresent invention include vinyl propionate, allyl acetate and allylpropionate.

Preferably, however, where the alkene is ethylene, the carboxylic acidused in the process of the present invention is acetic acid, such thatthe alkenyl carboxylate produced is vinyl acetate.

The carboxylic acid may be introduced into the reaction zone in liquidform or in vapour form. Where the process is a fixed bed process thenthe carboxylic acid is preferably introduced in to the reaction zone invapour form. Where the process is a fluid bed process then thecarboxylic acid is preferably introduced in to the reaction zone as aliquid spray.

The carboxylic acid may comprise fresh and/or recycle acid.

The fresh and recycle carboxylic acid may be introduced into thereaction zone either as separate feed streams or as a single feed streamcomprising both fresh and recycle acid.

The carboxylic acid may comprise at least a portion of the acid obtainedfrom downstream processes such as from the separation of the acid from amixture of the acid/alkenyl carboxylate/water.

The molecular oxygen-containing gas may be any suitable gas containingmolecular oxygen and may suitably be air or a gas richer or poorer inmolecular oxygen than air. A suitable molecular oxygen-containing gasmay be, for example, oxygen diluted with a suitable diluent, for examplenitrogen, argon or carbon dioxide. Preferably, the molecularoxygen-containing gas is essentially pure oxygen.

Under normal operating conditions the alkene, carboxylic acid andmolecular oxygen-containing gas may be introduced into the reaction zonein any suitable proportions for the production of the alkenylcarboxylate. For example, the alkene may be present in the feed to thereaction zone in a range between 30 and 85 mol % of the total reactioncomposition, preferably at least 50 mol %, such as in an amount of atleast 60 mol % of the total reaction composition. The carboxylic acidmay be present in the feed to the reaction zone in a range between 2 and30 mol % of the total reaction composition, preferably 5 to 15 mol %.The amount of molecular oxygen-containing gas present in the feed to thereaction zone is controlled by flammability constraints. In a fixed bedreactor the molecular oxygen-containing gas is preferably introduced tothe reaction zone via the recycle gas and the feed to the reaction zonemust be such that the mixture is non-flammable, for example oxygen maybe present in a range 3 to 9 mol % of the total reaction composition. Ina fluid bed reactor the molecular oxygen-containing gas is preferablyadded directly to the reaction zone and the oxygen may be present at ahigher level, for example, in a range 3 to 20 mol % of the totalreaction composition. A balance of an inert gas, preferably one or moreof nitrogen, carbon dioxide and argon may also be present in thereactant feed.

The catalyst for use in the process of the present invention may be anypromoted Group VIII metal suitable for the production of an alkenylcarboxylate from an alkene, carboxylic acid and a molecularoxygen-containing gas.

Where vinyl acetate is the alkenyl carboxylate, the catalyst suitablefor use in the production of vinyl acetate in a fixed bed process maycomprise any suitable catalyst known in the art, for example, asdescribed in GB 1 559 540 and U.S. Pat. No. 5,185,308.

GB 1 559 540 describes a catalyst active for the preparation of vinylacetate by the reaction of ethylene, acetic acid and molecular oxygen,the catalyst consisting essentially of:

(1) a catalyst support having a particle diameter of from 3 to 7 mm anda pore volume of from 0.2 to 1.5 ml/g, a 10% by weight water suspensionof the catalyst support having a pH from 3.0 to 9.0,

(2) a palladium-gold alloy distributed in a surface layer of thecatalyst support, the surface layer extending less than 0.5 mm from thesurface of the support, the palladium in the alloy being present in anamount of from 1.5 to 5.0 grams per litre of catalyst, and the goldbeing present in an amount of from 0.5 to 2.25 grams per litre ofcatalyst, and

(3) from 5 to 60 grams per litre of catalyst of alkali metal acetate.

U.S. Pat. No. 5,185,308 describes a shell impregnated catalyst activefor the production of vinyl acetate from ethylene, acetic acid and amolecular oxygen containing gas, the catalyst consisting essentially of:

(1) a catalyst support having a particle diameter from about 3 to about7 mm and a pore volume of 0.2 to 1.5 ml per gram,

(2) palladium and gold distributed in the outermost 1.0 mm thick layerof the catalyst support particles, and

(3) from about 3.5 to about 9.5% by weight of potassium acetate whereinthe gold to palladium weight ratio in said catalyst is in the range 0.6to 1.25.

A catalyst suitable for use in the production of vinyl acetate in afluid bed process may comprise a Group VIII metal, a catalyst promoterand an optional co-promoter.

With regards to the Group VIII metal, the preferred metal is palladium.Suitable sources of palladium include palladium (II) chloride, sodium orpotassium tetrachloropalladate, (II), (Na₂PdCl₄ or K₂PdCl₄), palladiumacetate, palladium (II) nitrate or palladium (II) sulphate. The metalmay be present in a concentration of greater than 0.2% by weight,preferably greater than 0.5% by weight based upon total weight ofcatalyst. The metal concentration may be as high as 10% by weight.Generally, the higher the active metal loading in a catalyst suitablefor use in vinyl acetate production, the more catalytically active itwill be. In addition to the Group VIII metal, the catalyst for theproduction of vinyl acetate comprises a promoter. Suitable promotersinclude gold, copper, cerium or mixtures thereof. A preferred promoteris gold. Suitable sources of gold include gold chloride,tetrachloroauric acid (HAuCl₄), NaAuCl₄, KAuCl₄, dimethyl gold acetate,barium acetoaurate or gold acetate. The preferred gold compound isHAuCl₄. The promoter metal may be present in an amount of from 0.1 to10% by weight in the finished catalyst.

The catalyst suitable for use in the production of vinyl acetate mayalso comprise a co-promoter material. Suitable co-promoters includeGroup I, Group II, lanthanide or transition metals, for example cadmium,barium, potassium, sodium, manganese, antimony, and/or lanthanum, whichare present in the finished catalyst as salts, e.g. an acetate salt. Thepreferred salts are potassium or sodium acetate. The co-promoter ispreferably present in the catalyst composition in a concentration of 0.1to 15% by weight of catalyst, more preferably, from 1 to 5% by weight.

Where a liquid acetic acid feed is used the preferred concentration ofco-promoter salt is up to 6% by weight, especially 2.5 to 5.5%. Wherethe acid is introduced in the vapour phase the co-promoter salt ispreferably present in a concentration up to 11 wt %.

The catalyst may be a supported catalyst. Suitable catalyst supportsinclude porous silica, alumina, silica/alumina, titania, silica/titaniaor zirconia. Where the catalyst is a catalyst suitable for use in theproduction of vinyl acetate, and in particular for use in a fluid bedprocess, the support is preferably silica, and, suitably, the supportmay have a pore volume from 0.2 to 3.5 ml per gram of support, a surfacearea of 5 to 800 m² per gram of support and an apparent bulk density of0.3 to 1.5 g/ml.

In a fluid bed reactor the particles of the catalyst are maintained in afluidised state by a suitable gas flow through the system. Excess flowrate may cause channeling of the gas through the reactor which decreasesconversion efficiency.

A typical catalyst useful in the production of vinyl acetate in afluidised bed reaction may have the following particle sizedistribution:

0 to 20 microns 0-30 wt % 20 to 44 microns 0-60 wt % 44 to 88 microns10-80 wt %  88 to 106 microns 0-80 wt % >106 microns 0-40 wt % >300microns  0-5 wt %

Persons skilled in the art will recognize that support particles sizesof 44, 88, 106 and 300 microns are arbitrary measures in that they arebased on standard sieve sizes. Particle sizes and particle sizedistributions may be measured by an automated laser device such as aMicrotrac X100.

The catalyst may be prepared by any suitable method. For example thecatalyst for the production of vinyl acetate may be prepared by themethod detailed in EP-A-0672453, the contents of which are herebyincorporated by reference.

The method of catalyst preparation may be varied to optimise catalystperformance based on maximising yield and selectivity.

The process of the present invention may be carried out at a temperaturein the reaction zone, T, from 100 to 400° C. and at atmospheric or atgreater than atmospheric pressure, for example, at up to 20 barg.

For example, the process for the production of vinyl acetate whencarried out in a fluid bed reaction zone may suitably be operated at atemperature from 100 to 400° C., preferably 140 to 210° C. and apressure of 1×10⁵ to 2×10⁶ Pa gauge (1 to 20 barg), preferably 6×10⁵ to1.5×10⁶ Pa gauge (6 to 15 barg), especially 7×10⁵ to 1.2×10⁶ Pa gauge (7to 12 barg).

The process for the production of vinyl acetate when carried out in afixed bed reaction zone may suitably be operated at a temperature from100 to 400° C., preferably 140 to 180° C. and a pressure of 1×10⁵ to2×10⁶ Pa gauge (1 to 20 barg), preferably 6×10⁵ to 1.5×10⁶ Pa gauge (6to 15 barg), especially 7×10⁵ to 1.2×10⁶ Pa gauge (7 to 12 barg).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be illustrated by reference to the followingExamples and Figures.

FIG. 1 is a graph illustrating the effect on catalytic activity ofpre-treating a vinyl acetate catalyst with ethylene or a mixture ofethylene and acetic acid, prior to contacting with oxygen.

FIG. 2 is a graph illustrating the effect on catalytic activity ofpre-treating a vinyl acetate catalyst with a mixture of ethylene andacetic acid, prior to contacting with oxygen.

FIG. 3 is a graph illustrating the effect on the oxygen concentration ofthe product stream from the reaction zone of a vinyl acetate fluidizedbed reactor by pre-treating the catalyst with ethylene or a mixture ofethylene and acetic acid.

FIG. 4 is a graph showing the increase in benzene production obtainedfrom a fluid bed vinyl acetate process as the oxygen concentration inthe product stream from the reaction zone is decreased.

EXAMPLES Example 1

These experiments demonstrate the effect on catalytic activity of:

-   1) contacting a vinyl acetate catalyst with ethylene prior to    contacting with oxygen-   2) contacting a vinyl acetate catalyst with a mixture of ethylene    and acetic acid prior to contacting with oxygen.

A 1.8 g sample of a promoted palladium vinyl acetate catalyst was mixedwith 20-22 g of inert diluent, and contacted for a period of 18, 66 or138 hours with either ethylene or a mixture of ethylene and acetic acid,at 160° C. and 8 barg in a fluidised bed microreactor.

The flow rates of ethylene and acetic acid were 0.49 mol/hr and 0.05mol/hr respectively. The fluidised bed microreactor had a diameter of 1″and was fitted with baffle trays. Gas flow was provided through a plenumat the base of the unit, and also from a small sparger mounted withinthe fluid bed.

After the pre-treatment with ethylene or a mixture of ethylene andacetic acid, each catalyst was tested in the fluidised bed microreactoroperated at 160° C./ and 8 bar.

A reaction mixture of 52mol % ethylene, 5 mol % acetic acid and 1.9mol %oxygen, with a nitrogen balance at a total flow rate of 0.93 mol/hr wasfed into the reactor. Samples were removed periodically and vinylacetate was measured by gas chromatography (GC).

The results are shown in FIGS. 1 and 2. FIG. 1 shows the activityprofile (space time yield (STY) in gVA/Kg-cat/hr) for a fresh, nonpre-treated catalyst (catalyst 1) compared to equivalent catalysts (2and 3) that have been pre-treated by exposure to ethylene and a mixtureof ethylene and acetic acid respectively for 138 hours. It can be seenthat both catalysts 2 and 3 have a significantly reduced initialactivity compared to the non-pretreated catalyst. With time on streamthe catalysts start to regain some of the lost activity.

It can also be seen that the catalyst exposed to a mixture of ethyleneand acetic acid (catalyst 3) is more severely inhibited than thatexposed to ethylene alone (catalyst 2).

FIG. 2 shows a comparison of the non pre-treated catalyst (catalyst 1,as above) with catalysts that have been pre-treated in a mixture ofethylene and acetic acid for two different periods of time, namely 18hours (catalyst 4) and 138 hours (catalyst 3, as above) respectively.

It can be seen that the initial extent of catalyst inhibition is relatedto the length of time for which the catalyst was exposed to the mixtureof ethylene and acetic acid.

Example 2

300 g samples of promoted palladium vinyl acetate catalysts werepre-treated for 18 hours (unless otherwise stated) in a fluidised bedreactor with a diameter of 1½″ (38 mm), fitted with baffle trays withthe materials as specified in Table 1. Gas flow was provided through aplenum at the base of the unit, and also from a small sparger mountedwithin the fluid bed. Heating was provided by a three-zone oil jacket,and the feed gases were pre-heated before entering the reactor. Thereactor was operated at 8 barg and 155° C. Nitrogen was used in allpre-treatments unless otherwise stated.

Catalyst Pre-treatment atmosphere 5 Ethylene, acetic acid, vinylacetate, nitrogen 6 Ethylene, nitrogen (with purge*) 7 Ethylene, aceticacid, nitrogen 8 Acetic acid, nitrogen 9 Ethylene, nitrogen *catalystpurged overnight with nitrogen after ethylene/nitrogen pre-treatment butbefore reaction.

After the pre-treatment the reactor was purged with nitrogen for 1 hour.A feed comprising 60 mol % ethylene, 12 mol % acetic acid, 6.8 mol %oxygen, balance nitrogen was then fed into the reactor. The oxygen flowin to the reactor was recorded in grammes per hour (g/hr) using a massflow controller. The oxygen level in the stream exiting the reactor wasmeasured using a Servomex oxygen analyser, which reported in weightpercent (wt %).

The results are shown in FIG. 3. FIG. 3 shows the oxygen levels exitingthe reactor for pre-treated catalysts 5 to 9 compared to the oxygenlevel in the feed to the reactor. Varying levels of inhibition can beobserved based on the time taken for the oxygen levels in the outlet todecrease relative to the oxygen feed level, denoting oxygen conversion.

As can be seen from FIG. 3, catalyst 7, pre-treated with ethylene,acetic acid and nitrogen was the most inhibited, and was more inhibitedthan a catalyst pre-treated with ethylene and nitrogen alone (catalysts6 and 9). The nitrogen purge used on catalyst 6 before start-up appearedto make little difference to the inhibition of this catalyst compared tocatalyst 9 which had had the same pre-treatment with ethylene andnitrogen, but without the subsequent nitrogen purge.

As comparison of catalyst 7 with catalyst 6 or catalyst 9 demonstrates,exposure to both acetic acid and ethylene inhibits the catalyst to agreater extent than exposure to ethylene alone. However, the catalystpre-treated with acetic acid and nitrogen only (catalyst 8) showedlittle or no inhibition compared to a fresh catalyst.

Example 3

A fluidised bed reactor was operated with varying levels of oxygen inthe product stream exiting from the reactor. The reactor was operated ata temperature of approximately 155° C., and at a pressure ofapproximately 7.5 barg. The feed to the reactor comprised ethylene,acetic acid and 3.5mol % oxygen. The oxygen conversion was adjusted togive an oxygen concentration in the reactor exit stream within the rangeof approximately 0.2 to 2.5 vol %. The oxygen concentration of thereactor exit stream was measured as an average over a 24 hour periodprior to taking a sample of the stream for benzene analysis.The results are shown in FIG. 4. It can be seen that benzene productionincreases significantly as the oxygen concentration in the reactor exitstream is reduced. In particular, below about 2 vol % oxygen over 100ppb of benzene is produced.

1. A fluid bed process for the production of an alkenyl carboxylatecomprising reacting an alkene, a carboxylic acid and a molecularoxygen-containing gas in a reaction zone in the presence of a catalystat an elevated reaction temperature, T, to produce an outlet stream fromthe reaction zone comprising alkenyl carboxylate and oxygen, whereinduring a process upset or shut-down, when the catalyst is contacted withthe alkene, at a partial pressure, P, optionally in the presence of thecarboxylic acid, and the outlet stream comprises less than 2 vol %oxygen, the partial pressure of the alkene is reduced to at least 50%less than P and/or the reaction temperature is reduced to below 100° C.so as to suppress formation of benzene and/or suppress inhibition of thecatalyst, the period of time, Z, during which the catalyst is exposed tothe alkene in the presence or absence of the carboxylic acid and atoxygen levels of less than 2 vol % oxygen before the partial pressure ofthe alkene is reduced to at least 50% less than P and/or the reactiontemperature is reduced to below 100° C. being in the range >0 to 12hours.
 2. A process as claimed in claim 1, wherein the catalyst iscontacted with alkene and carboxylic acid, and the outlet streamcomprises less than 2 vol % oxygen.
 3. A process as claimed in claim 1or claim 2, wherein the outlet stream comprises 0 to 0.5 vol % oxygen.4. A process as claimed in claim 1 or claIm 2, wherein the alkenylcarboxylate product comprises less than 100 ppb benzene.
 5. A process asclaimed in claim 1 or claIm 2, wherein the partial pressure of alkene,P, in the reaction zone is at least 0.3 bar or greater.
 6. A process asclaimed in claim 1 or claim 2, wherein the partial pressure of alkene inthe reaction zone is reduced by removing substantially all the alkenefrom the reaction zone.
 7. A process as claimed in claim 6, wherein thealkene, optional carboxylic acid, and any oxygen present, are removedfrom the reaction zone by purging the reaction zone with an inert gas.8. A process as claimed in claim 1 or claim 2, wherein the reactiontemperature is reduced to at least 20° C. below T.
 9. A process asclaimed in claim 1 or claim 2, wherein the catalyst comprises a GroupVIII metal, a promoter and optionally a co-promoter.
 10. A processaccording to claim 1, wherein the temperature in the reaction zone, T,is in the range 100° C.-400° C. and the pressure in the reaction zone isfrom atmospheric pressure up to 20 barg.
 11. A process as claimed inclaim 3, wherein the outlet stream comprises 0 to 0.2 vol % oxygen. 12.A process as claimed in claim 5, wherein the alkene, P, is ethylene andpartial pressure in the reaction zone is at least 1 bar.
 13. A processas claimed in claim 12, wherein the partial pressure in the reactionzone is at least 2 bar.
 14. A process as claimed in claim 7, wherein theinert gas is nitrogen.
 15. A process as claimed in claim 1 or claim 2,wherein the reaction is carried out at a temperature, T, of at least140° C.
 16. A process as claimed in claim 1 or claim 2, wherein thereaction temperature is reduced to 50° C. or lower.
 17. A process asclaimed in claim 8, wherein the reaction temperature is reduced to atleast 50° C. below T.
 18. A process as claimed in claim 1 or claim 2,wherein the catalyst is in contact with the alkene, and optionally thecarboxylic acid, at low levels of molecular oxygen, for >0 to 6 hoursprior to reducing the partial pressure of the alkene and/or reducing thereaction temperature.
 19. A process as claimed in claim 1 or claim 2,wherein the catalyst is in contact with the alkene and the carboxylicacid, at low levels of molecular oxygen, for >0 to 6 hours prior toreducing the partial pressure of the alkene and/or reducing the reactiontemperature.